As known in the art, practically all modems can be put into a local or remote loopback mode. In local loopback mode, signals are generated and transmitted to the local modem to test both the analog and digital circuits of that modem. These signals are then retransmitted back to the generating source for analysis to determine the quality and error rate of the data signal within the modem. In the remote loopback test, a device generates and transmits a signal to the local modem. The local modem demodulates the signals, then transmits it to the remote modem. If the remote modem is placed into the loopback mode, then the signal is passed through the analog circuitry and retransmitted back on the line to the local modem, which passes the signal on to the generating device. At that time, the generating device can then analyze the loopback signals for their quality and error rate.
The problem with loopback testing is that only the modems and the communication lines through which the data signals are sent are tested. Loopback testing does not test the local devices (e.g. telephone, fax machine or PC) attached to the modem. Nor does loopback testing test any devices connected to the user premises communication lines. Instead, for testing local plain old telephone system (POTS) devices, special diagnostic equipment is needed. This special diagnostic equipment operates to test a POTS device impedance over the modem spectrum (frequency bandwidth utilized), noise emanations, and distortions. Types of noise emanations that can be detected are background noise, impulse noise, noise with a tone, impedance, and non-linear distortion.
Noise with tone is an active noise that can be generated by elements in the communication circuits, such as comparators and quantizers. Many of these elements are active only when a tone is being transmitted. To test noise contributed by these elements, a tone must be generated and transmitted to measure the resultant signal. A notch filter may then be used to remove the test tone to allow analysis of the noise generated on the line.
Background noise is a form of constant signal on the communications line or in a communication circuit due to the random movement of electrons. In general, the power of background noise is proportional to the temperature.
Impulse noise can be a major cause of errors in any device or communication link. Impulse noise can be created by many different sources such as weather changes, dialing noise, dirty electrical contacts, and movement of poorly connected electronic devices.
Distortions come in numerous forms classified generally as either linear or non-linear distortion.
Linear gain and phase distortion are frequency dependent responses, which are not a function of signal amplitude, or phase and time invariant.
Another type of distortion is non-linear gain and phase distortion. Gain or phase delay are not constant, but vary non-linearly as a function of signal amplitude, and may include a frequency dependence.
Yet another observed form of non-linear distortion is intermodulation distortion. Intermodulation distortion involves the generation of new signal components that are not present in the original transmitted signal. Main causes of intermodulation distortion are electronic devices such as modulators, demodulators and overdriven amplifiers.
Another parameter tested in a device is the return loss or the impedance match of a circuit. Mismatch occurs in a device when its input impedance does not match a standard impedance or transmission line characteristic impedance.
Heretofore, modems have lacked the ability to test a device's impact on a user premises POTS system. Accordingly, there is a need to develop a system having the ability to identify whether a device connected to the POTS system within the user's premises may be limiting the achievable data rate of the modem. With such a system, any device identified as limiting the maximum achievable data rate of a multiple channel data communications device, could be disconnected from the premises wiring or connected to the premises wiring through a simple filter or isolating device to improve the modem operation.